In general, in an engine, a camshaft is rotated by torque of a crankshaft, external air is supplied into the combustion chamber by an intake valve and a fuel gas is injected into the combustion chamber while the intake and exhaust valves are reciprocated up/down at regular time interval by cams formed on the camshaft, a combustion gas is discharged by the exhaust valve by compressing and exploding a gas mixture, and a process of obtaining power from the explosive pressure is repeated.
FIG. 1 is a schematic cross-sectional view showing a valve train of a common vehicle.
A unit including a series of components such as a camshaft, a cam, a cam follower (or valve tappet), a push rod, a rocker arm, a valve spring, and a valve in order to operate intake and exhaust valves, as described above, is called a valve train.
FIG. 1 shows a valve train according to the related art, in which a plurality of cams 2 are formed at regular intervals along the axial line on a camshaft 1 and a cam follower 5 is disposed at the lower end of a push rod 4 that can slide up/down in an engine body block 3.
Further, the upper end of the push rod 4 is pivotably connected to a side of the rocker arm 6 and the upper end of the a valve 9 provided at an intake port or an exhaust port of a cylinder head block 7 and elastically supported by a valve spring 8 is pivotably connected to the other side of the locker arm 6.
The cam 2 of the camshaft 1 and the cam follower 5 of the push rod 4, which make a relative motion through the liquid lubricant while supporting load, have a small area at the friction portion in line contact with each other, such that large friction is generated under very high surface pressure between the cam 2 and the cam follower 5.
Therefore, in general, the two solid surfaces are not easily and completely separated only by the oil layer pressure of the lubricant, such that they are operated under composite lubrication including contact and lubrication or interface lubrication forming a surface layer through contact and lubrication. In general, the friction property is not good and a large amount of heat and wear is generated under the composite friction or the interface lubrication, and when a vehicle travels for a long time under those operating conditions, the lubrication surfaces of the cam 2 and the cam follower 5 may be damaged.
Meanwhile, it has been well known from a liquid lubrication theory that when the two surfaces are parallel, fluid dynamic pressure is not generated in lubrication even if the two surfaces make relative motion through the liquid lubricant. Though there is an exception, the fluid dynamic pressure is usually generated when a wedge effect reducing the thickness of an oil layer in the sliding direction. For example, in a dynamic pressure thrust bearing and a journal bearing, the thrust bearing and the journal bearing generate the wedge effect through an assembly error and eccentricity, respectively.
However, common workpieces have fine curves or surface curves due to surface roughness. Even if two surfaces relatively move in parallel with each other, there are areas where oil layer thickness locally reduces in the sliding direction and the oil layer pressure generated in the areas improve lubrication performance between the two surfaces. On the contrary, there are also areas where the oil layer thickness increases in the sliding direction, where bubbles are usually generated in the areas and pressure similar to the peripheral pressure is generated.
Therefore, when a plurality of fine prominences and depressions is formed on at least one of two surfaces making relative motion, fluid dynamic pressure is generated between the two surfaces and the lubrication performance can be correspondingly improved, even if the two surfaces relatively move in parallel with each other. Further, it has been known that the fine prominences and depressions catch worn particles or function as fine oil storage, such that the technology has been studied in various fields due to the effects.
The point of the technology of reducing friction and wear due to fine prominences and depressions on a surface is to determine the shape of the prominences and depressions and the arranging method such that friction and wear become minimized. However, since the shape of the prominences and depressions and the arranging method are greatly influenced by the operating conditions such as the contact type of two surfaces, load, and sliding speed, there is large difficulty in developing the technology. For example, the shape of the prominences and depressions and the arranging method for minimizing friction and wear are changed in accordance with the type of the contact portion, that is, a line type, a point type, and a surface type. Therefore, it is necessary to define first the operation environment or the operating conditions in order to develop the technology of surface prominence and depression for reducing friction and wear, and it is necessary to develop the shape of the prominences and depressions and the arrangement under the determined operation environment and the operating conditions.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.